LONDON — Boris Berezovsky, a self-exiled and outspoken Russian tycoon who had a bitter falling out with Russian President Vladimir Putin, was found dead in southeast England on Saturday. He was 67.

In recent years, the one-time Kremlin powerbroker-turned-thorn in Putin’s side fended off attacks in cases that often bore political undertones — and bit into his fortune.

Last year he lost what was billed as the largest private lawsuit in history, an epic tug of war over $5.1 billion with another Russian tycoon, Roman Abramovich, in which legal and other costs were estimated to be $250 million.

The cause of Mr. Berezovsky’s death was not clear, and Thames Valley police said it was being treated as “unexplained.” The police would not directly identify him, but when asked about Mr. Berezovsky by name they read a statement saying they were investigating the death of a 67-year-old man at a property in Ascot, about 25 miles west of London.

Lawyer Alexander Dobrovinsky told Russian state TV that his client — who had survived assassination attempts in the past — lately had been in “a horrible, terrible” emotional state. “All he had was debts,” Dobrovinsky told Russian state TV. “He was practically destroyed. He was selling his paintings and other things.”

Dobrovinsky wrote in Russian on his own Facebook page: “Just got a call from London. Boris Berezovsky has committed suicide. The man was complex. An act of desperation? Impossible to live poor? A series of blows? I am afraid that no one will know the truth.”

A mathematician-turned-Mercedes dealer, Mr. Berezovsky amassed his wealth during Russia’s chaotic privatization of state assets in the early 1990s after the collapse of the Soviet Union. In return for backing former Russian President Boris Yeltsin, he gained political clout and opportunities to buy state assets at knockdown prices, making a fortune in oil and automobiles.

He also played a key role in the rise of Yeltsin’s successor, Putin, in 2000. But Mr. Berezovsky later fell out of favor with Putin, and eventually sought political asylum in Britain in the early 2000s to evade fraud charges he contended were politically motivated.

Mr. Berezovsky was one of several so-called Russian oligarchs to butt heads with Putin.

After coming into power, the Russian president effectively made a pact: the oligarchs could keep their money if they didn’t challenge him politically. Those who refused often found themselves in dire circumstances. Some were imprisoned — such as former Yukos Oil chief Mikhail Khodorkovsky — while others, such as Mr. Berezovsky, fled Russia.

The assets of these pariah businessmen, meanwhile, were acquired by state corporations or cooperative tycoons, often at bargain prices.

Over the years, Mr. Berezovsky accused Putin of leading Russia toward dictatorship and returning it to a Soviet-style system of state monopoly on the media.

In Britain, Mr. Berezovsky allied himself with an array of other Kremlin critics. Among them was ex-KGB agent Alexander Litvinenko, who fled Russia with Mr. Berezovsky’s help after accusing officials there of plotting to assassinate political opponents.

Litvinenko died on Nov. 26, 2006, after drinking tea laced with a lethal dose of the rare radioactive isotope polonium-210 in a London hotel. From his deathbed, Litvinenko accused the Kremlin of orchestrating his poisoning, and British police named former KGB agent Andrei Lugovoi as the prime suspect.

Lugovoi and the Kremlin denied the accusations, with the former instead claiming that Mr. Berezovsky — whom Russia repeatedly sought to extradite on a wide variety of criminal charges — engineered Litvinenko’s death to embarrass the Kremlin and buttress his refugee status.

Mr. Berezovsky consistently denied the allegations. In 2010, he won a libel case against Kremlin-owned broadcaster All-Russian State Television and Radio Broadcasting, which aired a show in which it was suggested he was behind the former agent’s poisoning.

Mr. Berezovsky, too, was the target of assassination attempts. In 1994, a car bomb injured him and killed his driver. He also said he briefly fled Britain in 2007 when British intelligence services told him his life was in danger.

“I was informed by Scotland Yard that there was a plot to kill me, and they recommended to me to leave the country,” he said at the time. Scotland Yard later arrested a man on suspicion of conspiring to murder the tycoon.

More recently, Mr. Berezovsky has made headlines for costly legal battles that have dealt serious blows to his finances.

Last year, the Russian business magnate was ordered to pay $53.3 million in legal costs to Abramovich, the billionaire owner of Chelsea Football Club, after losing the multimillion-dollar legal battle.

Mr. Berezovsky had claimed Abramovich cheated him out of his stakes in the oil group Sibneft, arguing that he blackmailed him into selling the stakes vastly beneath their true worth after he lost Putin’s good graces.

A judge threw out the case in August, ruling that Mr. Berezovsky was a dishonest and unreliable witness, and rejected his claims that he was threatened by Putin and Alexander Voloshin, a Putin ally, to coerce him to sell his Sibneft stake.

It also recently emerged that Mr. Berezovsky ran up legal bills totaling more than $380,000 in two months in a case against his former partner, Elena Gorbunova, with whom he had two children and who claimed the businessman owed her millions.

Last week, The Times of London newspaper reported that Mr. Berezovsky was selling property — including an Andy Warhol portrait of Vladimir Lenin — to settle his debts and pay expenses owed to lawyers.

The Russian president’s spokesman, Dmitry Peskov, said in a telephone interview on state television that Mr. Berezovsky had sent a letter to Putin about two months ago asking to be allowed to return to Russia. In the letter, Mr. Berezovsky acknowledged having made many mistakes, Peskov said.